This invention is concerned with fuel systems for internal combustion engines. A typical fuel system comprises a tank, a fuel pump delivering fuel from the tank to a carburetor or other device for atomizing the fuel and mixing that fuel with air, and an intake conduit leading from the carburetor or other device to the combustion chamber of the engine or, in a multi-cylinder engine, more usually to a manifold which distributes the fuel and air mixture to the cylinders.
It is recognized that to improve engine efficiency it is desirable to obtain vaporization of the fuel or at least as fine an atomization as is possible and to this end, it has been proposed to heat the fuel/air mixture prior to its delivery to the combustion chambers of the engine. Heating of the mixture has generally been achieved by passing that mixture in heat exchanging relationship with the exhaust gases of the engine, with the engine lubricant or with the engine coolant.
A typical arrangement is that illustrated in Canadian Pat. No. 1,070,197 issued Jan. 22, 1980 to Nissan Motor Company, Limited of Japan. In that patent there is disclosed a system in which a heat exchange chamber is formed in the intake manifold of the engine and engine coolant is directed to that chamber from the water jacket of the engine. A problem arises with such prior art arrangements during start-up periods when the engine is cold. During these periods, rather than improving vaporization or gasification of the fuel, the heat exchanger will tend to cause the fuel to be condensed or cause individual droplets of fuel in the mixture to coalesce thus seriously impairing engine efficiency. To alleviate this problem, it has been necessary to use a very simple heat exchanger having a relatively small heat exchanging surface, and one in which the path followed by the fuel is not substantially increased over the path which it would follow if the heat exchanger was not present. This technique, while sacrificing the most efficient preheating of the fuel air mixture for normal hot operation of the engine, reduces the area over which the fuel may condense and the length of the path over which the droplets of fuel may coalesce, during cold operation of the engine. This technique has represented at best a compromise between the preheating which is optimum to obtain vaporization during normal hot operation of the engine, and the need to avoid condensation or coalescing of the fuel during cold operation.